Exhaust emission control system for internal combustion engine

ABSTRACT

An exhaust emission control system for an internal combustion engine is disclosed. The system includes a NOx removing device provided in an exhaust system of the engine for removing NOx contained in exhaust gases, and an oxygen concentration sensor provided in the exhaust system. The air-fuel ratio of the air-fuel mixture to be supplied to the engine is changed from a value which is leaner than the stoichiometric ratio to a value which is richer than the stoichiometric ratio. It is determined whether or not a sulfur oxide concentration in the exhaust gases is high according to a transient characteristic of an oxygen concentration detected by the oxygen concentration sensor after the air-fuel ratio is changed.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an exhaust emission controlsystem for an internal combustion engine, and more particularly to suchan exhaust emission control system having a function of monitoring theconcentration of sulfur oxide (SOx) contained in exhaust gases.

[0002] Conventionally known is a technique for removing NOx (nitrogenoxide) contained in exhaust gases by providing a NOx removing deviceincluding a NOx absorbent in an exhaust system of an internal combustionengine. Further, a technique of determining a degree of deterioration ofthe NOx removing device is known in the art (Japanese Patent Laid-openNo. Hei 10-299460). In this technique, two oxygen concentration sensorsare arranged upstream and downstream of the NOx removing device, and theair-fuel ratio of an air-fuel mixture to be supplied to the engine ischanged from a lean region to a rich region with respect to thestoichiometric ratio. Then, the degree of deterioration of the NOxremoving device is determined according to a delay time period from thetime an output from the upstream oxygen concentration sensor has changedto a value indicative of the rich air-fuel ratio to the time an outputfrom the downstream oxygen concentration sensor has changed to a valueindicative of the rich air-fuel ratio.

[0003] However, when the concentration of SOx in the vicinity of eachoxygen concentration sensor becomes high, an output characteristic ofthe oxygen concentration sensor changes. Accordingly, when theconcentration of SOx in the vicinity of each oxygen concentration sensoris high, there is a case that the deterioration determination of the NOxremoving device cannot be accurately performed. When the degree ofenrichment of the air-fuel ratio is low, that is, when the air-fuelratio after enrichment is near the stoichiometric ratio, the effect ofSOx tends to become especially remarkable.

SUMMARY OF THE INVENTION

[0004] It is accordingly an object of the present invention to providean exhaust emission control system which can determine a condition wherethe SOx concentration in the vicinity of each oxygen concentrationsensor is high.

[0005] In order to attain the above object, the present inventionprovides an exhaust emission control system for an internal combustionengine, including NOx removing means, an oxygen concentration sensor,air-fuel ratio changing means, and sulfur oxide determining means. TheNOx removing means is provided in an exhaust system of the engine, andremoves NOx contained in exhaust gases when the air-fuel ratio of anair-fuel mixture to be supplied to the engine is set to a value leanerthan a stoichiometric ratio. The oxygen concentration sensor is providedin the exhaust system and detects an oxygen concentration in the exhaustgases. The air-fuel ratio changing means changes the air-fuel ratio ofthe air-fuel mixture to be supplied to the engine from a value which isleaner than the stoichiometric ratio to a value which is richer than thestoichiometric ratio. The sulfur oxide determining means determineswhether or not a sulfur oxide concentration in the exhaust gases is highaccording to a transient characteristic of the oxygen concentrationdetected by the oxygen concentration sensor after the air-fuel ratio ischanged by the air-fuel ratio changing means.

[0006] More specifically, the condition where the sulfur oxideconcentration is high corresponds to a condition that the sulfur oxideconcentration in the exhaust gases is high enough to possibly have aneffect on the output from the oxygen concentration sensor.

[0007] With this configuration, the air-fuel ratio is changed from avalue which is leaner than the stoichiometric ratio to a value which isricher than the stoichiometric ratio, and it is determined whether ornot the sulfur oxide concentration in the exhaust gases is highaccording to the transient characteristic of the detected oxygenconcentration, after changing the air-fuel ratio as mentioned above. Ithas been experimentally confirmed that when the sulfur oxideconcentration in the exhaust gases becomes high, the time period duringwhich the oxygen concentration sensor output changes from a valueindicative of a lean air-fuel ratio to a value indicative of a richair-fuel ratio tends to increase or the oxygen concentration sensoroutput tends to change toward a lean region after once reaching a valueindicative of a rich air-fuel ratio. By detecting such a tendency, thecondition where the sulfur oxide concentration is high can bedetermined.

[0008] Preferably, the air-fuel ratio changing means changes theair-fuel ratio from a value which is leaner than the stoichiometricratio to a value which is slightly richer than the stoichiometric ratio.The sulfur oxide determining means determines that the sulfur oxideconcentration is high when a transient time period from the time theoxygen concentration detected by the oxygen concentration sensor hasbecome lower than a first reference value, to the time the oxygenconcentration detected by the oxygen concentration sensor reaches asecond reference value, which is less than the first reference value, islonger than a predetermined transient time period.

[0009] With this configuration, the air-fuel ratio is changed from avalue which is leaner than the stoichiometric ratio to a value which isslightly richer than the stoichiometric ratio. It is determined that thesulfur oxide concentration is high when the transient time period fromthe time the detected oxygen concentration has become lower than thefirst reference value to the time the detected oxygen concentrationreaches the second reference value which is less than the firstreference value, is longer than the predetermined transient time period.That is, the tendency that the time period during which the oxygenconcentration sensor output changes from a value indicative of a leanair-fuel ratio to a value indicative of a rich air-fuel ratio increases,is detected, whereby the condition that the sulfur oxide concentrationis high can be determined.

[0010] Preferably, the sulfur oxide determining means determines thatthe sulfur oxide concentration is high, when the oxygen concentrationdetected by the oxygen concentration sensor becomes lower than aconcentration determination reference value, and thereafter exceeds theconcentration determination reference, value within a predetermined timeperiod from the time the oxygen concentration becomes lower than theconcentration determination reference value.

[0011] With this configuration, it is determined that the sulfur oxideconcentration is high when the detected oxygen concentration exceeds theconcentration determination reference value within the predeterminedtime period, from the time the oxygen concentration becomes lower thanthe concentration determination reference value. That is, the tendencythat the oxygen concentration sensor output changes toward a lean regionafter once reaching a value indicative of a rich air-fuel ratio, isdetected; whereby the condition that the sulfur oxide concentration ishigh can be determined.

[0012] Preferably, the air-fuel ratio changing means changes theair-fuel ratio from a value which is leaner than the stoichiometricratio to a value which is slightly richer than the stoichiometric ratio.

[0013] With this configuration, by setting the degree of enrichment ofthe air-fuel ratio with respect to the stoichiometric ratio to a smallvalue, the oxygen concentration sensor output can be easily affected bythe sulfur oxide. This improves the determination accuracy.

[0014] Preferably, the exhaust emission control system further includesdeterioration determining means and inhibiting means. The deteriorationdetermining means determines the deterioration of the NOx removingdevice according to the output from the oxygen concentration sensor. Theinhibiting means inhibits the deterioration determination by thedeterioration determining means when the sulfur oxide determining meansdetermines that the sulfur oxide concentration is high.

[0015] With this configuration, the deterioration of the NOx removingmeans is determined according to the oxygen concentration sensor outputby the deterioration determining means. The deterioration determinationby the deterioration determining means is inhibited by the inhibitingmeans, when the sulfur oxide determining means determines that thesulfur oxide concentration is high. If the deterioration determinationaccording to the oxygen concentration sensor output is performed in thecondition where the effect of the sulfur oxide is large, there is apossibility of improper determination. Accordingly, by inhibiting thedeterioration determination in such a case, improper determination canbe prevented.

[0016] Preferably, the exhaust emission control system further includessulfur oxide removing means. The sulfur oxide removing means executes aprocess for removing sulfur oxide accumulated in the NOx removing meanswhen the sulfur oxide determining means determines that the sulfur oxideconcentration is high.

[0017] With this configuration, the process for removing the sulfuroxide accumulated in the NOx removing means is executed, when the sulfuroxide determining means determines that the sulfur oxide concentrationis high. Accordingly, it is possible to prevent improper determinationsuch that a reduction in performance of the NOx removing means, due tothe accumulation of the sulfur oxide, is improperly determined as anaged deterioration of the NOx removing means.

[0018] The present invention provides another exhaust emission controlsystem for an internal combustion engine, including a three-waycatalyst, NOx removing means, an oxygen concentration sensor, air-fuelratio changing means, and sulfur oxide determining means. The three-waycatalyst is provided in an exhaust system of the engine and purifiesexhaust gases. The NOx removing means is provided downstream of thethree-way catalyst and removes NOx contained in the exhaust gases whenthe air-fuel ratio of an air-fuel mixture to be supplied to the engineis set to a value leaner than a stoichiometric ratio. The oxygenconcentration sensor is provided between the three-way catalyst and theNOx removing means and detects an oxygen concentration in the exhaustgases. The air-fuel ratio changing means changes the air-fuel ratio ofthe air-fuel mixture from a value which is leaner than thestoichiometric ratio to a value which is richer than the stoichiometricratio. The sulfur oxide determining means determines whether or not asulfur oxide concentration in the exhaust gases is high according to atransient characteristic of the oxygen concentration detected by theoxygen concentration sensor, after the air-fuel ratio is changed by theair-fuel ratio changing means.

[0019] With this configuration, the air-fuel ratio is changed from avalue which is leaner than the stoichiometric ratio to a value which isricher than the stoichiometric ratio, and it is determined whether ornot the sulfur oxide concentration in the exhaust gases is highaccording to the transient characteristic of the detected oxygenconcentration, after changing the air-fuel ratio as mentioned above. Ithas been experimentally confirmed that when the three-way catalystprovided upstream of the oxygen concentration sensor is deteriorated,the sulfur oxide concentration on the downstream side of the three-waycatalyst becomes high. As a result, the time period during which theoxygen concentration sensor output changes from a value indicative of alean air-fuel ratio to a value indicative of a rich air-fuel ratio tendsto increase or the oxygen concentration sensor output tends to changetoward a lean region after once reaching a value indicative of a richair-fuel ratio. By detecting such a tendency, the condition where thesulfur oxide concentration on the downstream side of the three-waycatalyst is high can be determined.

[0020] Preferably, the sulfur oxide determining means determines thatthe sulfur oxide concentration is high, when the oxygen concentrationdetected by the oxygen concentration sensor becomes lower than a firstreference value and fails to reach a second reference value, which isless than the first reference value, within a predetermined time periodelapsed from the time the oxygen concentration becomes lower than thefirst reference value.

[0021] With this configuration, it is determined that the sulfur oxideconcentration is high when the detected oxygen concentration fails toreach the second reference value, which is less than the first referencevalue, within the predetermined time period elapsed from the time thedetected oxygen concentration has become lower than the first referencevalue. That is, the tendency that the time period during which theoxygen concentration sensor output changes from a value indicative of alean air-fuel ratio to a value indicative of a rich air-fuel ratioincreases is detected; whereby the condition where the sulfur oxideconcentration is high can be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic diagram showing the configuration of aninternal combustion engine and a control system therefor according to afirst preferred embodiment of the present invention;

[0023]FIG. 2 is a flowchart showing a program for setting a targetair-fuel ratio coefficient (KCMD);

[0024]FIG. 3 is a flowchart showing a main routine for executingdeterioration determination for a NOx removing device;

[0025]FIG. 4 is a flowchart showing a program for determining executionconditions of the deterioration determination;

[0026]FIG. 5 is a flowchart showing a program for determining executionconditions of the deterioration determination;

[0027]FIG. 6 is a flowchart showing a program for determining a SOxconcentration;

[0028]FIGS. 7A to 7C are time charts for illustrating the process ofFIGS. 4 and 5 and the process of FIG. 6;

[0029]FIG. 8 is a flowchart showing a subroutine for executing thedeterioration determination for the NOx removing device;

[0030]FIG. 9 is a flowchart showing a subroutine for executing thedeterioration determination for the NOx removing device;

[0031]FIGS. 10A to 10K are time charts for illustrating the process ofFIGS. 8 and 9;

[0032]FIG. 11 is a flowchart showing a program for calculating anexhaust amount parameter (GALNCS) in the process of FIG. 8;

[0033]FIG. 12 is a graph showing a table used in the process of FIG. 11;

[0034]FIG. 13 is a time chart for illustrating the deteriorationdetermination for the NOx removing device;

[0035]FIG. 14 is a flowchart showing a program for removing SOx;

[0036]FIG. 15 is a flowchart showing a program for inhibiting a leanoperation;

[0037]FIG. 16 is a schematic diagram showing the configuration of aninternal combustion engine and a control system therefor according to asecond preferred embodiment of the present invention; and

[0038]FIG. 17 is a flowchart showing a program for calculating anexhaust amount parameter (GALNCS) in the second preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Some preferred embodiments of the present invention will now bedescribed with reference to the drawings.

FIRST PREFERRED EMBODIMENT

[0040] Referring to FIG. 1, there is schematically shown a generalconfiguration of an internal combustion engine (which will behereinafter referred to as “engine”) and a control system therefor,including an exhaust emission control system according to a firstpreferred embodiment of the present invention. The engine is afour-cylinder engine 1, for example, and it has an intake pipe 2provided with a throttle valve 3. A throttle valve opening (THA) sensor4 is connected to the throttle valve 3, so as to output an electricalsignal corresponding to an opening of the throttle valve 3 and supplythe electrical signal to an electronic control unit (which will behereinafter referred to as “ECU”) 5 for controlling the engine 1.

[0041] Fuel injection valves 6, only one of which is shown, are insertedinto the intake pipe 2 at locations intermediate between the cylinderblock of the engine 1 and the throttle valve 3 and slightly upstream ofthe respective intake valves (not shown). These fuel injection valves 6are connected to a fuel pump (not shown), and electrically connected tothe ECU 5. A valve opening period of each fuel injection valve 6 iscontrolled by a signal output from the ECU 5.

[0042] An absolute intake pressure (PBA) sensor 8 is providedimmediately downstream of the throttle valve 3. An absolute pressuresignal converted to an electrical signal by the absolute intake pressuresensor 8, is supplied to the ECU 5. An intake air temperature (TA)sensor 9 is provided downstream of the absolute intake pressure sensor 8to detect an intake air temperature TA. An electrical signalcorresponding to the detected intake air temperature TA, is outputtedfrom the sensor 9 and supplied to the ECU 5.

[0043] An engine coolant temperature (TW) sensor 10 such as a thermistoris mounted on the body of the engine 1 to detect an engine coolanttemperature (cooling water temperature) TW. A temperature signalcorresponding to the detected engine coolant temperature TW is outputfrom the sensor 10 and supplied to the ECU 5.

[0044] An engine rotational speed (NE) sensor 11 and a cylinderdiscrimination (CYL) sensor 12 are mounted in facing relation to acamshaft or a crankshaft (both not shown) of the engine 1. The enginerotational speed sensor 11 outputs a TDC signal pulse at a crank angleposition located at a predetermined crank angle before the top deadcenter (TDC) corresponding to the start of an intake stroke of eachcylinder of the engine 1 (at every 180° crank angle in the case of afour-cylinder engine). The cylinder discrimination sensor 12 outputs acylinder discrimination signal pulse at a predetermined crank angleposition for a specific cylinder of engine 1. These signal pulses outputfrom the sensors 11 and 12 are supplied to the ECU 5.

[0045] An exhaust pipe 13 of the engine 1 is provided with a three-waycatalyst 14 and a NOx removing device 15, as NOx removing means,arranged downstream of the three-way catalyst 14.

[0046] The three-way catalyst 14 has an oxygen storing capacity, and hasthe function of storing some of the oxygen contained in the exhaustgases in the exhaust lean condition where the air-fuel ratio of anair-fuel mixture to be supplied to the engine 1 is set in a lean region,with respect to the stoichiometric ratio, and the oxygen concentrationin the exhaust gases is therefore relatively high. The three-waycatalyst 14 also has the function of oxidizing HC and CO contained inthe exhaust gases by using the stored oxygen in the exhaust richcondition where the air-fuel ratio of the air-fuel mixture to besupplied to the engine 1 is set in a rich region, with respect to thestoichiometric ratio, and the oxygen concentration in the exhaust gasesis therefore low with a large proportion of HC and CO components.

[0047] The NOx removing device 15 includes a NOx absorbent for absorbingNOx and a catalyst for accelerating oxidation and reduction. The NOxremoving device 15 absorbs NOx in the exhaust lean condition where theair-fuel ratio of the air-fuel mixture to be supplied to the engine 1 isset in a lean region with respect to the stoichiometric ratio. The NOxremoving device 15 discharges the absorbed NOx in the exhaust richcondition, where the air-fuel ratio of the air-fuel mixture supplied toengine 1 is in the vicinity of the stoichiometric ratio or in a richregion with respect to the stoichiometric ratio, thereby reducing thedischarged NOx into nitrogen gas by HC and CO and oxidizing the HC andCO into water vapor and carbon dioxide.

[0048] When the amount of NOx absorbed by the NOx absorbent reaches thelimit of its NOx absorbing capacity, i.e., the maximum NOx absorbingamount, the NOx absorbent cannot absorb any more NOx. Accordingly, todischarge the absorbed NOx and reduce it, the air-fuel ratio isenriched, that is, reduction enrichment of the air-fuel ratio isperformed.

[0049] A proportional type air-fuel ratio sensor (hereinafter referredto as “LAF sensor”) 17 is mounted on the exhaust pipe 13 at a positionupstream of the three-way catalyst 14. The LAF sensor 17 outputs anelectrical signal substantially proportional to the oxygen concentration(air-fuel ratio) in the exhaust gases, and supplies the electricalsignal to the ECU 5.

[0050] A binary type oxygen concentration sensor (hereinafter referredto as “O2 sensor”) 18 is mounted on the exhaust pipe 13 at a positionbetween the three-way catalyst 14 and the NOx removing device 15, and anO2 sensor 19 is mounted on the exhaust pipe 13 at a position downstreamof the NOx removing device 15. Detection signals from sensors 18 and 19are supplied to the ECU 5. Each of the O2 sensors 18 and 19 has acharacteristic such that its output rapidly changes in the vicinity ofthe stoichiometric ratio. More specifically, each of the sensors 18 and19 outputs a high level signal in a rich region with respect to thestoichiometric ratio, and outputs a low level signal in a lean regionwith respect to the stoichiometric ratio.

[0051] The engine 1 has a valve timing switching mechanism 30 capable ofswitching the valve timing of intake valves and exhaust valves between ahigh-speed valve timing suitable for a high-speed operating region ofthe engine 1 and a low-speed valve timing suitable for a low-speedoperating region of the engine 1. This switching of the valve timingalso includes switching of a valve lift amount. Further, when selectingthe low-speed valve timing, one of the two intake valves in eachcylinder is stopped to ensure stable combustion even in the case ofsetting the air-fuel ratio lean with respect to the stoichiometricratio.

[0052] The valve timing switching mechanism 30 is of such a type thatthe switching of the valve timing is carried out hydraulically. That is,a solenoid valve for performing the hydraulic switching and an oilpressure sensor are connected to the ECU 5. A detection signal from theoil pressure sensor is supplied to the ECU 5, and the ECU 5 controls thesolenoid valve to perform the switching control of the valve timingaccording to an operating condition of the engine 1.

[0053] An atmospheric pressure sensor 20 for detecting an atmosphericpressure PA is connected to the ECU 5, and a detection signal from theatmospheric pressure sensor 20 is supplied to the ECU 5.

[0054] The ECU 5 includes an input circuit 5a having various functionsincluding a function of shaping the waveforms of input signals from thevarious sensors, a function of correcting the voltage levels of theinput signals to a predetermined level, and a function of convertinganalog signal values into digital signal values, and a centralprocessing unit (hereinafter referred to as “CPU”) 5 b. A memory circuit5 c consisting of a ROM (read only memory) preliminarily stores variousoperational programs to be executed by the CPU 5 b, and a RAM (randomaccess memory) for storing the results of computations or the like bythe CPU 5 b, and an output circuit 5 d for supplying drive signals tothe fuel injection valves 6.

[0055] The CPU 5 b determines various engine operating conditionsaccording to various engine operating parameter signals as mentionedabove, and calculates a fuel injection period TOUT of each fuelinjection valve 6 to be opened in synchronism with the TDC signal pulse,in accordance with Eq. (1) according to the above determined engineoperating conditions.

TOUT=TIM×KCMD×KLAF×KPA×K 1+K 2   (1)

[0056] TIM is a basic fuel amount, more specifically, a basic fuelinjection period of each fuel injection valve 6, and it is determined byretrieving a TI map set according to the engine rotational speed NE andthe absolute intake pressure PBA. The TI map is set so that the air-fuelratio of an air-fuel mixture to be supplied to the engine 1 becomessubstantially equal to the stoichiometric ratio in an operatingcondition according to the engine rotational speed NE and the absoluteintake pressure PBA. That is, the basic fuel amount TIM has a valuesubstantially proportional to an intake air amount (mass flow) per unittime by the engine.

[0057] KCMD is a target air-fuel ratio coefficient, which is setaccording to engine operational parameters such as the engine rotationalspeed NE, the throttle valve opening THA, and the engine coolanttemperature TW. The target air-fuel ratio coefficient KCMD isproportional to the reciprocal of an air-fuel ratio A/F, i.e.,proportional to a fuel-air ratio F/A, and takes a value of 1.0 for thestoichiometric ratio, therefore, KCMD is referred to also as a targetequivalent ratio. Further, in the case of executing reduction enrichmentor determination of deterioration of the NOx removing device 15 to behereinafter described, the target air-fuel ratio coefficient KCMD is setto a predetermined enrichment value KCMDRR or KCMDRM for enrichment ofan air-fuel ratio.

[0058] KLAF is an air-fuel ratio correction coefficient calculated byPID (Proportional Integral Differential) control so that a detectedequivalent ratio KACT calculated from a detected value from the LAFsensor 17 becomes equal to the target equivalent ratio KCMD, in the casethat the conditions for execution of feedback control are satisfied.

[0059] KPA is an atmospheric pressure correction coefficient setaccording to the atmospheric pressure PA. When the atmospheric pressurePA is in the vicinity of 101.3 kPa, KPA is set to “1.0” (noncorrectivevalue). When the atmospheric pressure PA decreases, KPA is set to avalue greater than “1.0,” thereby correcting the fuel supply amount inits increasing direction. As will be hereinafter described, theatmospheric pressure correction coefficient KPA is set so as to increasewith a decrease in the atmospheric pressure PA, and the fuel supplyamount is corrected so as to increase with a decrease in the atmosphericpressure PA.

[0060] K1 and K2 are respectively a correction coefficient and acorrection variable computed according to various engine parametersignals. The correction coefficient K1 and correction variable K2 arepredetermined values that optimize various characteristics such as fuelconsumption characteristics and engine acceleration characteristics,according to engine operating conditions.

[0061] The CPU 5 b supplies a drive signal for opening each fuelinjection valve 6 according to the fuel injection period TOUT obtainedabove, through the output circuit 5 d to the fuel injection valve 6.

[0062]FIG. 2 is a flowchart showing a program for calculating the targetair-fuel ratio coefficient KCMD applied to Eq. (1) mentioned above. Thisprogram is executed by the CPU 5 b at predetermined time intervals.

[0063] In step S10, it is determined whether or not a SOx removalenrichment flag FSRR set in the process, (SOx removal process) shown inFIG. 14 described below, is “1.” When the SOx removal enrichment flagFSRR is set to “1,” it indicates the execution of air-fuel ratioenrichment for removing SOx accumulated in the NOx removing device 15.If FSRR is “0” in step S10, the program proceeds to step S11. If FSRR is“1” in step S10, the target air-fuel ratio coefficient KCMD is set to apredetermined value KCMDSF (e.g., 1.03) for SOx removal enrichment (stepS29).

[0064] In step S11, it is determined whether or not the engine 1 is in alean operating condition, that is, whether or not a stored value KCMDBof the target air-fuel ratio coefficient KCMD stored in step S20described below during normal control is less than “1.0.” If KCMDB isgreater than or equal to 1.0, that is, if the engine 1 is not in thelean operating condition, the program proceeds directly to step S16, inwhich a reduction enrichment flag FRSPOK is set to “0.” The flag FRSPOKset to “1” indicates the duration of execution of reduction enrichment.Thereafter, a reduction enrichment time TRR (e.g., 5 to 10 sec) is setto a downcount timer tmRR, to be referred in step S23 described below,and a deterioration determination enrichment time TRM (>TRR), which islonger than the reduction enrichment time TRR, is set to a downcounttimer tmRM, to be referred to in step S27 described below. Then, thetimers tmRR and tmRM are started (step S17).

[0065] Thereafter, it is determined whether or not an enrichmentcontinuation flag FRSPEXT is “0” (step S18). The enrichment continuationflag FRSPEXT is set by the process shown in FIG. 9 described below. Theflag FRSPEXT set to “1” indicates the continuation of air-fuel ratioenrichment even after ending the deterioration determination for the NOxremoving device 15. If FRSPEXT is “1,” the program proceeds to step S26described below, in which the air-fuel ratio enrichment is continued.

[0066] If FRSPEXT is “0” in step S18, normal control is performed to setthe target air-fuel ratio coefficient KCMD according to engine operatingconditions (step S19). Basically, the target air-fuel ratio coefficientKCMD is calculated according to the engine speed NE and the absoluteintake pressure PBA. However, in the condition where the engine coolanttemperature TW is low or in a predetermined high-load operatingcondition, the set value of the target air-fuel ratio coefficient ischanged according to these conditions. Then, the target air-fuel ratiocoefficient KCMD calculated in step S19 is stored as a stored valueKCMDB (step S20), and this program ends. In an engine operatingcondition where the lean operation is allowed, the target air-fuel ratiocoefficient KCMD is set to a value which is less than “1.0.”

[0067] If KCMDB is less than “1.0” in step S11, that is, if the engine 1is in the lean operating condition, an increment value ADDNOx to be usedin step S13 is decided according to the engine speed NE and the absoluteintake pressure PBA (step S12). The increment value ADDNOx is aparameter corresponding to the amount of NOx exhausted per unit timeduring the lean operation, and this parameter increases with an increasein the engine speed NE and with an increase in the absolute intakepressure PBA.

[0068] In step S13, the increment value ADDNOx decided in step S12 isapplied to the following expression to increment a NOx amount counterCRSP, thereby obtaining a NOx exhaust amount, that is, a count valuecorresponding to the amount of NOx absorbed by the NOx absorbent, asshown in Eq. (2).

CRSP=CRSP+ADDNOx   (2)

[0069] In step S14, it is determined whether or not an executioncondition flag FMCNDF105 is “1.” When the execution condition ofdeterioration determination of the NOx removing device 15 is satisfied,the execution condition flag FMCNDF105 is set to “1.” Normally,FMCNDF105 is “0,” so that the program proceeds to step S15. In step S15,it is determined whether or not the current value of the NOx amountcounter CRSP has exceeded an allowable value CNOxREF. If the answer tostep S15 is negative (NO), the program proceeds to step S16, in whichthe normal control is performed, that is, the target air-fuel ratiocoefficient KCMD is set according to engine operating conditions unlessthe enrichment continuation flag FRSPEXT is set to “1.” The allowablevalue CNOxREF is set to a value corresponding to an NOx amount slightlyless than the maximum NOx absorption amount of the NOx absorbent, forexample.

[0070] If CRSP is greater than CNOxREF in step S15, the reductionenrichment flag FRSPOK is set to “1” (step S21). Subsequently, thetarget air-fuel ratio coefficient KCMD is set to a predeterminedenrichment value KCMDRR corresponding to a value equivalent to anair-fuel ratio of 14.0, thus executing reduction enrichment (step S22).Then, it is determined whether or not the current value of the timertmRR is “0” (step S23). If tmRR is greater than “0,” this program endsat once. If tmRR is “0,” the reduction enrichment flag FRSPOK is set to“0” and the current value of the NOx amount counter CRSP is reset to “0”(step S24). Accordingly, the answer to step S15 becomes negative (NO),so that the normal control is then performed.

[0071] If the deterioration determination execution condition issatisfied (FMCNDF105 is “1”), the program proceeds from step S14 to stepS26, in which the target air-fuel ratio coefficient KCMD is set to apredetermined deterioration determination enrichment value KCMDRM(<KCMDRR) corresponding to a value (e.g., air-fuel ratio of 14.3)slightly shifted to the lean region from a value equivalent to anair-fuel ratio of 14.0, thus executing deterioration determinationenrichment (step S26). The reason for making the degree of enrichmentsmaller in the execution of deterioration determination than the degreeof enrichment in the usual reduction enrichment is that if the degree ofenrichment is large, the enrichment execution time is short, andimproper determination is prone to occur at the time of deteriorationdetermination of the NOx removing device 15. Accordingly, by reducingthe degree of enrichment and increasing the enrichment execution time(=TRM), the accuracy of deterioration determination can be improved.Furthermore, by reducing the degree of enrichment, the outputs from theO2 sensors 18 and 19 become susceptible to SOx, thereby improving theaccuracy of determining a condition of high SOx concentration.

[0072] Subsequently, it is determined whether or not the current valueof the timer tmRM is “0” (step S27). If tmRM is greater than “0,” thisprogram ends at once. If tmRM is “0,” the current value of the NOxamount counter CRSP is reset to “0” (step S28).

[0073] According to the process shown in FIG. 2, the reductionenrichment is executed intermittently in an engine operating conditionwhere the lean operation is allowed (steps S22 and S23), so that NOxabsorbed by the NOx absorbent in the NOx removing device 15 is suitablyreduced. Further, when the deterioration determination executioncondition for the NOx removing device 15 is satisfied, the deteriorationdetermination enrichment is executed so that the degree of enrichment ismade smaller than the degree of enrichment in the reduction enrichmentand that the execution time period TRM is made longer than the executiontime period of the reduction enrichment (steps S26 and S27). Further, inthe case of executing the SOx removal (see FIG. 14), the SOx removalenrichment is executed (steps S10 and S29). Further, when the enrichmentcontinuation flag FRSPEXT is set to “1” in the process shown in FIG. 9(step S123) described below, the target air-fuel ratio coefficient KCMDis maintained at the predetermined enrichment value KCMDRM even aftertermination of the deterioration determination for the NOx removingdevice 15, to continue the air-fuel ratio enrichment (steps S18 andS26).

[0074]FIG. 3 is a flowchart showing a main routine of the deteriorationdetermination process for the NOx removing device 15. This process isexecuted by the CPU 5 b in synchronism with the generation of a TDCsignal pulse.

[0075] In step S32, the execution condition determination process shownin FIGS. 4 and 5 is executed. In the execution condition determinationprocess, it is determined whether or not the execution condition ofdeterioration determination for the NOx removing device 15 is satisfied.If the execution condition is satisfied, the execution condition flagFMCNDF105 is set to “1.” In step S33, it is determined whether or notthe execution condition flag FMCNDF105 is “1.” If FMCNDF105 is “0,”which indicates that the execution condition is not satisfied, a SOxconcentration determination end flag FSLFEND, a first reference overflag FSVO2EXPL, and a second reference over flag FSVO2EXPH are all setto “0” (steps S34, S35, and S36), and this process ends.

[0076] The SOx concentration determination end flag FSLFEND is set to“1” when the SOx concentration determination process shown in FIG. 6ends. The first reference over flag FSVO2EXPL is set to “1” when theupstream O2 sensor output SVO2 has reached a first upstream referencevalue SVO2LNCS (e.g., 0.3 V) (step S82 in FIG. 6). The second referenceover flag FSVO2EXPH is set to “1” when the upstream O2 sensor outputSVO2 has exceeded a second upstream reference value SVO2SLF (e.g., 0.7V) which is higher than the first upstream reference value SVO2LNCS(step S88 in FIG. 6).

[0077] If the execution condition flag FMCNDF105 is “1” in step S33,which indicates that the execution condition is satisfied, the SOxconcentration determination process shown in FIG. 6 is executed (stepS37), and the deterioration determination subroutine shown in FIGS. 8and 9 is next executed (step S38).

[0078]FIGS. 4 and 5 are flowcharts of the execution conditiondetermination process executed in step S32 shown in FIG. 3.

[0079] In step S41, it is determined whether or not a deteriorationdetermination command flag FGOF105 is “1.” It is sufficient that thedeterioration determination of the NOx removing device 15 is to beexecuted once in one operational period (a period from engine startingto stopping). Therefore, the deterioration determination command flagFGOF105 is set to “1” at the time the engine operating condition becomesstable, after starting of the engine. If the deterioration determinationcommand flag FGOF105 is set to “1,” it is determined whether or not adeterioration determination end flag FENDF105 is “1” (step S42).

[0080] If the answer to step S41 is negative (NO) or the answer to stepS42 is affirmative (YES), the deterioration determination end flagFENDF105 is set to “0” (step S43), and the program proceeds to step S49.

[0081] If the answer to step S42 is negative (NO), it is determinedwhether or not an upstream O2 sensor activation flag FNSO2 is “1” (stepS44). The flag FNSO2 set to “1” indicates that the upstream O2 sensor 18has been activated. If the answer to step S44 is affirmative (YES), itis determined whether or not a downstream O2 sensor activation flagFNLO2 is “1” (step S45). The flag FNLO2 set to “1” indicates that thedownstream O2 sensor 19 has been activated.

[0082] If the answer to step S45 is affirmative (YES), it is determinedwhether or not a lean operation flag FLB is “1” (step S46). The leanoperation flag FLB set to “1” indicates that the lean operation in whichthe air-fuel ratio is set leaner than the stoichiometric ratio isallowed (the engine is in an operating condition where the leanoperation can be performed). If the answer to step S46 is affirmative(YES), it is determined whether or not the reduction enrichment flagFRSPOK is “1” (step S47).

[0083] If the answer to any one of steps S44 to S46 is negative (NO) orthe answer to step S47 is affirmative (YES), a precondition flag FLNCMWTis set to “0” (step S49). After executing step S49, the program proceedsto step S53.

[0084] On the other hand, if the answers to all of steps S44 to S46 areaffirmative (YES) and the answer to step S47 is negative (NO), that is,if the activation of the O2 sensors 18 and 19 has been completed, thelean operation is allowed and the air-fuel ratio reduction enrichment isnot executed, and the precondition flag FLNCMWT is set to “1” (stepS48).

[0085] In step S50, it is determined whether or not the value of the NOxamount counter CRSP is greater than a deterioration determinationallowance value CLNCMACT. The deterioration determination allowancevalue CLNCMACT is set to a value less than or equal to the allowablevalue CNOxREF used in the process shown in FIG. 2. If CRSP is less thanor equal to CLNCMACT in step S50, the program proceeds to step S53, inwhich a failure determination condition flag FMCDF103B is set to “0”.The failure determination condition flag FMCDF103B set to “1” indicatesthat the execution condition of failure determination process (notshown) for the downstream O2 sensor 19 is satisfied.

[0086] If CRSP is greater than CLNCMACT in step S50, which indicatesthat NOx has been absorbed in the NOx removing device 15 by an amountwhich is enough to execute the deterioration determination of the NOxremoving device 15, it is determined whether or not an O2 sensor failuredetermination end flag FDONEF103 is “1” (step S51). If FDONEF103 is “1,”which indicates that the failure determination for the O2 sensor 19 hasended, the program proceeds directly to step S54. If FDONEF103 is “0,”the failure determination condition flag FMCDF103B is set to “1” (stepS52), and the program proceeds to step S54.

[0087] In step S54, it is determined whether or not the executioncondition flag FMCNDF105 has already been set to “1.” If FMCNDF105 is“1,” the program proceeds directly to step S57. If FMCNDF105 is “0,” itis determined whether or not the upstream O2 sensor output SVO2 is lessthan or equal to a third upstream reference value SVO2LNCM (e.g., 0.1V), which is less than the above-mentioned first upstream referencevalue SVO2LNCS (step S55). If SVO2 is less than or equal to SVO2LNCM, itis determined whether or not the downstream O2 sensor output LVO2 isless than or equal to a first downstream reference value LVO2LNCM, whichis set to a value substantially equal to the third upstream referencevalue SVO2LNCM (step S56). Steps S55 and S56 are executed to check thateach of the O2 sensor outputs SVO2 and LVO2, before execution of thedeterioration determination enrichment, is a value indicative of anexhaust lean condition (a condition where the oxygen concentration inthe exhaust gases is relatively high).

[0088] As the result, if the answer to step S55 or S56 is negative (NO),a purge cut flag FLNCPG is set to “0” (step S58), and a downcount timerTLNCPG is set to a predetermined time TMLNCPG (e.g., 2 seconds) andstarted (step S59). Thereafter, a rich zone flag FSLFZONE is set to “0”(step S61), a maximum value parameter SVMAXLNC is set to “0” (step S62),and the execution condition flag FMCNDF105 is set to “0” (step S63).Then, this program ends.

[0089] The purge cut flag FLNCPG set to “1” indicates that theevaporative fuel purge for supplying evaporative fuel generated in thefuel tank to the intake pipe 2 is inhibited. The rich zone flag FSLFZONEis set to “1” when the upstream O2 sensor output SVO2 becomes greaterthan or equal to the second upstream reference value SVO2SLF (see stepsS64 and S65). The maximum value parameter SVMAXLNC is a parameterindicative of a maximum value before the upstream O2 sensor output SVO2reaches the second upstream reference value SVO2SLF (see steps S66 toS68).

[0090] If the answers to both of steps S55 and S56 are affirmative(YES), which indicates that both of the upstream O2 sensor output SVO2and the downstream O2 sensor output LVO2 indicate the exhaust leancondition, the purge cut flag FLNCPG is set to “1” (step S57).Thereafter, it is determined whether or not the value of the timerTLNCPG started in step S59 is “0” (step S60). If TLNCPG is greater than“0,” the program proceeds to step S61.

[0091] When the value of the timer TLNCPG becomes “0,” the programproceeds from step S60 to step S64, in which it is determined whether ornot the upstream O2 sensor output SVO2 is less than the second upstreamreference value SVO2SLF. Initially, the answer to step S64 isaffirmative (YES), so that the program skips step S65 to go to step S66,in which it is determined whether or not the rich zone flag FSLFZONE is“1.” Initially, the answer to step S66 is negative (NO), so that theprogram proceeds to step S67, in which it is determined whether or notthe upstream O2 sensor output SVO2 is greater than the maximum valueparameter SVMAXLNC. Since the maximum value parameter SVMAXLNC isinitialized to “0” in step S62, the answer to step S67 is initiallyaffirmative (YES). Then, the maximum value parameter SVMAXLNC is set toa current value of the O2 sensor output SVO2 (step S68), and the programproceeds to step S71. In step S71, the execution condition flagFMCNDF105 is set to “1.”

[0092] When the O2 sensor output SVO2 increases monotonically, theanswer to step S67 is always affirmative (YES). However, when there is acase that the O2 sensor output SVO2 temporarily decreases as shown inFIG. 7A, the answer to step S67 becomes negative (NO) and the programproceeds to step S69. In step S69, a difference DSV between the maximumvalue parameter SVMAXLNC and the O2 sensor output SVO2 is calculatedfrom the following equation Eq. (3):

DSV=SVMAXLNC−SVO2   (3)

[0093] It is then determined whether or not the difference DSV isgreater than a predetermined difference DSVLNCMC (step S70). If thedifference DSV exceeds the predetermined difference DSVLNCMC, theprogram proceeds to step S63, in which the execution condition flagFMCNDF105 is set to “0”.

[0094] In the above case, when the difference DSV exceeds thepredetermined difference DSVLNCMC, it is considered that the exhaustlean condition has been temporarily produced because of engineacceleration or the like. If the deterioration determination iscontinued in this case, there is a possibility of improperdetermination. Therefore, it is decided that the execution condition isnot satisfied in this case to suspend the deterioration determination.

[0095] If the answer to step S70 does not become affirmative (YES), butthe O2 sensor output SVO2 reaches the second upstream reference valueSVO2SLF, the rich zone flag FSLFZONE is set to “1” (step S65), and theprogram proceeds from step S66 directly to step S71.

[0096] According to the process of FIGS. 4 and 5, the executioncondition of deterioration determination for the NOx removing device 15is basically satisfied when the precondition flag FLNCMWT is set to “1.”However, when the exhaust condition either in the vicinity of theupstream O2 sensor or in the vicinity of the downstream O2 sensor is notthe exhaust lean condition (steps S55 and S56), the execution conditionis not satisfied. Further, during the predetermined time period TMLNCPGfrom the time of inhibition of the evaporative fuel purge, the executioncondition is not satisfied (steps S57 and S60). Further, when thetemporary decrease (DSV) of the upstream O2 sensor output SVO2 becomesgreater than the predetermined difference DSVLNCMC in the conditionwhere the upstream O2 sensor output SVO2 is lower than the secondupstream reference value SVO2SLF (when the answer to step S70 isaffirmative (YES)), the execution condition is not satisfied.

[0097]FIG. 6 is a flowchart of the SOx concentration determinationprocess executed in step S37 shown in FIG. 3.

[0098] In step S81, it is determined whether or not the upstream O2sensor output SVO2 is greater than or equal to the first upstreamreference value SVO2LNCS. Initially, SVO2 is less than SVO2LNCS, so thatthe program skips step S82 to go to step S83. In step S83, it isdetermined whether or not the first reference over flag FSVO2 EXPL,which is set to “1” in step S82, is “1.” Initially, the answer to stepS83 is negative (NO), so that a first exhaust amount parameter GSLFFINis set to “0” (step S84), and the program proceeds to step S87.

[0099] In step S87, it is determined whether or not the O2 sensor outputSVO2 has exceeded the second upstream reference value SVO2SLF.Initially, the answer to step S87 is negative (NO), so that the programskips step S88 to go to step S89. In step S89, it is determined whetheror not the second reference over flag FSVO2EXPH, which is set to “1” instep S88, is “1.” Initially, the answer to step S89 is negative (NO), sothat a second exhaust amount parameter GSLFJUD is set to “0” (step S90),and a minimum value parameter SVO2 MIN is set to a maximum value VMAX(e.g., in the case that one byte is allocated to the minimum valueparameter SVO2MIN, a hexadecimal FF is set as the maximum value VMAX)(step S91). Thereafter, the program proceeds to step S95.

[0100] In step S95, it is determined whether or not the second exhaustamount parameter GSLFJUD is greater than or equal to a seconddetermination threshold GASLF. Initially, the answer to step S95 isnegative (NO), so that it is determined whether or not the first exhaustamount parameter GSLFFIN is greater than or equal to a firstdetermination threshold GSLFFINR (step S96). Initially, the answer tostep S96 is also negative (NO), so that this process ends.

[0101] When the O2 sensor output SVO2 reaches the first upstreamreference value SVO2LNCS, the first reference over flag FSVO2EXPL is setto “1” (step S82), and the program proceeds from step S83 to step S85.In step S85, the first exhaust amount parameter GSLFFIN is calculatedfrom Eq. (4) shown below.

GSLFFIN=GSLFFIN+(TIM×KPA)   (4)

[0102] In the Eq. (4), GSLFFIN on the right side is a value calculatedin the preceding execution of this process. TIM and KPA are the basicfuel amount and the atmospheric pressure correction coefficient shown inEq. (1), respectively. TIM is a basic fuel amount, that is, a fuelamount set so that the air-fuel ratio becomes the stoichiometric ratioaccording to engine operating conditions (engine speed NE and absoluteintake pressure PBA). Accordingly, TIM is a parameter which isproportional to an intake air amount per unit time of the engine 1. Inother words, TIM is a parameter which is proportional to an amount ofexhaust gases per unit time of the engine 1. Thus, the first exhaustamount parameter GSLFFIN, corresponding to an accumulated value of theamount of exhaust gases flowing into the NOx removing device 15 from thetime the upstream O2 sensor output SVO2 has reached the first upstreamreference value SVO2LNCS, is obtained by the calculation of Eq. (4).

[0103] During the execution of the deterioration determination, theair-fuel ratio is maintained at a fixed rich air-fuel ratio (a valuecorresponding to KCMDRM) in a rich region with respect to thestoichiometric ratio. Therefore, the exhaust amount parameter GSLFFINhas a value proportional to an integrated value of the amounts ofreducing components (HC and CO) contained in the exhaust gases. Further,the exhaust amount parameter GSLFFIN is proportional to the time elapsedfrom the time of starting the integration if the engine operatingcondition is substantially constant. These points apply similarly to theother exhaust amount parameters described below.

[0104] If the O2 sensor output SVO2 falls between the first upstreamreference value SVO2LNCS and the second upstream reference valueSVO2SLF, the program proceeds from step S87 through steps S89, S90, S91,and S95 to step S96. If the first exhaust amount parameter GSLFFIN isless than the first determination threshold GSLFFINR, this process ends.If the first exhaust amount parameter GSLFFIN reaches the firstdetermination threshold GSLFFINR, a high concentration flag FSLF is setto “1” (step S98), and the program proceeds to step S99.

[0105] When the O2 sensor output SVO2 is less than the second upstreamreference value SVO2SLF, at the time the first exhaust amount parameterGSLFFIN has reached the first determination threshold GSLFFINR, as shownby the solid line in FIG. 7B, it is determined that the SOxconcentration in the vicinity of the O2 sensor 18 is high.

[0106] When the SOx concentration is high, there are two cases. A firstcase is that the time period until the O2 sensor output SVO2 reaches thesecond upstream reference value SVO2SLF is long. A second case is thatthe O2 sensor output SVO2 stays at a value lower than the secondupstream reference value SVO2SLF. According to steps S85 and S96, thehigh SOx concentration can be determined in both the first and secondcases mentioned above.

[0107] If SVO2 is greater than SVO2SLF in step S87, the second referenceover flag FSVO2EXPH is set to “1” (step S88), and the program proceedsfrom step S89 to step S92. In step S92, the second exhaust amountparameter GSLFJUD is calculated from Eq. (5) shown below.

GSLFJUD=GSLFJUD+(TIM×KPA)   (5)

[0108] Eq. (5) is obtained by substituting GSLFJUD for GSLFFIN in Eq.(4). The second exhaust amount parameter GSLFJUD, which corresponds toan accumulated value of the amount of exhaust gases flowing into the NOxremoving device 15 from the time the upstream O2 sensor output SVO2 hasexceeded the second upstream reference value SVO2LNCS, is obtained bythe calculation of Eq. (5) (see FIG. 7B).

[0109] In step S93, it is determined whether or not the minimum valueparameter SVO2MIN is greater than the O2 sensor output SVO2. Initially,SVO2MIN is greater than SVO2. Accordingly, the minimum value parameterSVO2MIN is set to a current value of the O2 sensor output SVO2 (stepS94), and the program proceeds to step S95. According to steps S93 andS94, the minimum value of the O2 sensor output SVO2, after setting thesecond reference over flag FSVO2EXPH to “1,” is calculated as theminimum value parameter SVO2MIN.

[0110] Until the second exhaust amount parameter GSLFJUD reaches thesecond determination threshold GASLF, the program proceeds to step S96.

[0111] If the second exhaust amount parameter GSLFJUD reaches the seconddetermination threshold GASLF, the program proceeds from step S95 tostep S97, in which it is determined whether or not the minimum valueparameter SVO2MIN is greater than or equal to the second upstreamreference value SVO2SLF. If SVO2MIN is greater than or equal to SVO2SLF,the concentration determination end flag FSLFEND is set to “1” (stepS99), and this process ends. If SVO2MIN is less than SVO2SLF, that is,if the O2 sensor output SVO2 exceeds the second upstream reference valueSVO2SLF, and thereafter becomes lower than the second upstream referencevalue SVO2SLF as shown in FIG. 7C, it is indicated that the saturatedoutput from the O2 sensor 18 tends to decrease. Accordingly, the highconcentration flag FSLF is set to “1” (step S98).

[0112] According to the process of FIG. 6, it is determined that the SOxconcentration is high when the O2 sensor output SVO2 changes, as shownby the solid line in FIG. 7B, or changes as shown in FIG. 7C. When theSOx concentration is high in the vicinity of the O2 sensor, it has beenexperimentally confirmed that the saturated output from the O2 sensortends to decrease. By detecting this tendency in the process of FIG. 6,the condition where the SOx concentration is high can be detected. Thehigh SOx concentration condition specifically corresponds to a conditionwhere the SOx concentration is about 600 PPM or more. In such acondition, the oxygen concentration sensor output changes under theinfluence of SOx.

[0113] Further, when the three-way catalyst is deteriorated, the SOxconcentration on the downstream side of the three-way catalyst tends toincrease. Accordingly, when the NOx removing device is located on thedownstream side of the three-way catalyst, as in this preferredembodiment, the SOx concentration on the downstream side of thethree-way catalyst increases due to the deterioration of the three-waycatalyst, causing a change in the oxygen concentration sensor output. Asa result, the accuracy of deterioration determination of the NOxremoving device is reduced. To cope with this problem, the deteriorationdetermination for the NOx removing device is suspended when the SOxconcentration is high as will be hereinafter described, which makes itpossible to prevent improper determination.

[0114] The tendency that the saturated output from the O2 sensordecreases appears more remarkably when the degree of the air-fuel ratioenrichment is lower. Accordingly, the target air-fuel ratio coefficientKCMD in the deterioration determination is set to the deteriorationdetermination enrichment predetermined value KCMDRM corresponding to anair-fuel ratio (e.g., about 14.3), which is slightly richer than thestoichiometric ratio in this preferred embodiment.

[0115]FIGS. 8 and 9 are flowcharts showing the deteriorationdetermination subroutine executed in step S38 shown in FIG. 3.

[0116] In step S101, it is determined whether or not the enrichmentcontinuation flag FRSPEXT is “1.” If FRSPEXT is “1,” the programproceeds directly to step S121. If FRSPEXT is “0,” it is determinedwhether or not a first prejudgment flag FPREJUD1 is “1” (step S102).Initially, the first prejudgment flag FPREJUD1 is “0,” since the flagFPREJUD1 is set to “1” in step S108. Accordingly, the program proceedsfrom step S102 to step S103, in which GALNCS calculation process shownin FIG. 11 is executed.

[0117] In step S141 shown in FIG. 11, it is determined whether or notthe upstream O2 sensor output SVO2 is less than or equal to a fourthupstream reference value SVO2LNCH (e.g., 0.6 V). If SVO2 is less than orequal to SVO2LNCH, a third exhaust amount parameter GALNCS is set to “0”(step S142), and the process of FIG. 11 ends.

[0118] If the O2 sensor output SVO2 exceeds the fourth upstreamreference value SVO2LNCH (see the solid line L1 in FIG. 13), KNACPBSshown in FIG. 12 is retrieved according to the absolute intake pressurePBA to calculate an intake pressure correction coefficient KNACPBS (stepS144). KNACPBS is set so that the intake pressure correction coefficientKNACPBS decreases with an increase in the absolute intake pressure PBA.

[0119] In step S145, the third exhaust amount parameter GALNCS iscalculated from Eq. (6) shown below.

GALNCS=GALNCS+(TIM×KPA×KNACPBS)   (6)

[0120] Eq. (6) is different from Eq. (5) in the point that the secondterm on the right side is multiplied by the intake pressure correctioncoefficient KNACPBS. It has been experimentally confirmed that theexhaust amount during the time period between adjacent TDC signal pulsesdecreases with an increase in the absolute intake pressure PBA. For thepurpose of correcting this point, the intake pressure correctioncoefficient KNACPBS is introduced.

[0121] By the calculation of Eq. (6), the third exhaust amount parameterGALNCS, which corresponds to an accumulated value of the amount ofexhaust gases flowing into the NOx removing device 15 from the time (t12shown in FIG. 13) the upstream O2 sensor output SVO2 has exceeded thefourth upstream reference value SVO2LNCH, is obtained.

[0122] Referring back to FIG. 8, in step S104, it is determined whetheror not the third exhaust amount parameter GALNCS calculated in step S103is greater than or equal to a third determination threshold GALNCHOK. IfGALNCS is less than GALNCHOK, the program proceeds directly to stepS109. If GALNCS is greater than or equal to GALNCHOK (time t14 in FIG.13), it is determined whether or not the downstream O2 sensor outputLVO2 is less than or equal to a second downstream reference valueLVO2LNCH (e.g., 0.7 V) (step S105).

[0123] If LVO2 is less than or equal to LVO2LNCH, as shown by the solidline L3 in FIG. 13, it is determined that the NOx removing device 15 isnormal. Then, a first prejudgment OK flag FOK105P is set to “1” and afirst prejudgment NG flag FNG105P is set to “0” (step S106). Thereafter,the program proceeds to step S108. On the other hand, if LVO2 is greaterthan LVO2LNCH, as shown by the broken line L2 in FIG. 13, it isdetermined that the NOx removing device 15 is deteriorated. Then, thefirst prejudgment NG flag FNG105P is set to “1” and the firstprejudgment OK flag FOK105P is set to “0” (step S107). Thereafter, theprogram proceeds to step S108.

[0124] In step S108, the first prejudgment flag FPREJUD1 is set to “1,”so as to indicate that the first prejudgment has been completed.

[0125] In step S109, it is determined whether or not a SOx removal endflag FSRMOVEND is “1.” The flag FSRMOVEND is set to “1” when the SOxremoval process shown in FIG. 14 has been completed. If FSRMOVEND is“0,” which indicates that the SOx removal process has not beencompleted, the program proceeds to step S118, and a second prejudgmentflag FPREJUD2 is set to “1” without substantially executing a secondprejudgment (step S118). Thereafter, the program proceeds to step S119.

[0126] If FSRMOVEND is “1” in step S109, which indicates that the SOxremoval process has been completed, it is determined whether or not thesecond prejudgment flag FPREJUD2 is “1” (step S110). If FPREJUD2 is “1,”a fourth exhaust amount parameter GAIRLVO2 is set to “0” (step S112),and the program proceeds to step S119.

[0127] If FPREJUD2 is “0” in step S10, it is determined whether or notthe downstream O2 sensor output LVO2 is greater than or equal to a thirddownstream reference value LVO2LNC (e.g., 0.3 V) (step S111). If theanswer to step S111 is negative (NO), the program proceeds to step S112.If the answer to step S111 is affirmative (YES), the fourth exhaustamount parameter GAIRLVO2 is calculated from Eq. (7) shown below (stepS113).

GAIRLVO2=GAIRLVO2+(TIM×KPA)   (7)

[0128] Thereafter, it is determined whether or not the fourth exhaustamount parameter GAIRLVO2 is greater than or equal to a fourthdetermination threshold GALVO2 (step S114). If GAIRLVO2 is less thanGALVO2, the program proceeds directly to step S119. If GAIRLVO2 isgreater than or equal to GALVO2, it is determined whether or not thedownstream O2 sensor output LVO2 is greater than or equal to a fourthdownstream reference value LVO2SLF (e.g., 0.7 V) (step S115).

[0129] If LVO2 is greater than or equal to LVO2SLF, it is determinedthat the NOx removing device 15 is normal. Then, a second prejudgment OKflag FOK105S is set to “1” and a second prejudgment NG flag FNG105S isset to “0” (step S116). Thereafter, the program proceeds to step S118.On the other hand, if LVO2 is less than LVO2SLF, it is determined thatthe NOx removing device 15 is deteriorated. Then, the second prejudgmentNG flag FNG105S is set to “1” and the second prejudgment OK flag FOK105Sis set to “0” (step S117). Thereafter, the program proceeds to stepS118.

[0130] In step S118, the second prejudgment flag FPREJUD2 is set to “1,”so as to indicate that the second prejudgment has been completed.

[0131] In step S119, it is determined whether or not the firstprejudgment flag FPREJUD1 is “1.” If FPREJUD1 is “1,” it is determinedwhether or not the second prejudgment flag FPREJUD2 is “1” (step S120).If the first prejudgment flag FPREJUD1 or the second prejudgment flagFPREJUD2 is “0,” this process ends. If the second prejudgment flagFPREJUD2 is “1,” the program proceeds from step S120 to step S121.

[0132] In step S121, it is determined whether or not an O2 sensorfailure flag FFSDF103 is “1.” The flag FFSDF103 is set to “1” when thedownstream O2 sensor 19 is determined to fail. If the answer to stepS121 is affirmative (YES), that is, if the downstream O2 sensor 19 isdetermined to fail, the program proceeds directly to step S135, in whichthe deterioration determination end flag FENDF105 is set to “1.”Further, all of the execution condition flags, FMCNDF105, the firstprejudgment flag FPREJUD1, and the second prejudgment flag FPREJUD2 areset to “0”. Then, this process ends. Accordingly, the deteriorationdetermination process is suspended.

[0133] If FFSDF103 is “0” in step S121, which indicates that thedownstream O2 sensor 19 is determined to fail, it is determined whetheror not an O2 sensor OK flag FOKF103 is “1” (step S122). The O2 sensor OKflag FOKF103 is set to “1” when the downstream O2 sensor 19 isdetermined to be normal. If FOKF103 is “0,” which indicates that thedownstream O2 sensor 19 is not determined to be normal, the enrichmentcontinuation flag FRSPEXT is set to “1,” so as to continue the air-fuelratio enrichment for executing the failure determination of thedownstream O2 sensor 19 (step S123). Thereafter, this process ends.

[0134] If FOKF103 is “1,” which indicates that the downstream O2 sensor19 is determined to be normal, it is determined whether or not theconcentration determination end flag FSLFEND is “1” (step S124). IfFSLFEND is “0,” this process ends. If FSLFEND is “1,” which indicatesthat the SOx concentration determination is completed, it is determinedwhether or not the high concentration flag FSLF is “1” (step Si 25).

[0135] If FSLF is “0,” which indicates that the SOx concentration islow, the SOx removal end flag FSRMOVEND is set to “0” (step S126). Next,it is determined whether or not the first prejudgment NG flag FNG105P is“1” (step Si 27). If the answer to step S127 is affirmative (YES), it isdetermined that the NOx removing device 15 is deteriorated. Then, thedeterioration flag FFSDF105 is set to “1,” the normality flag FOKF105 isset to “0,” and the end flag FDONEF105 is set to “1” (step S134).Thereafter, the program proceeds to step S135. If the answer to stepS127 is negative (NO), it is determined whether or not the firstprejudgment OK flag FOK105P is “1” (step S128). If the answer to stepS128 is negative (NO), this process ends. If the answer to step S128 isaffirmative (YES), it is determined that the NOx removing device 15 isnormal. Then, the normality flag FOKF105 is set to “1,” thedeterioration flag FFSDF105 is set to “0,” and the end flag FDONEF105 isset to “1” (step S133). Thereafter, the program proceeds to step S135.

[0136] If FSLF is “1” in step S125, which indicates that the SOxconcentration is high, it is determined whether or not the SOx removalend flag FSRMOVEND is “1” (step S129). If FSRMOVEND is “0,” whichindicates that the SOx removal process has not been completed, theprogram proceeds directly to step S135 to end the deteriorationdetermination.

[0137] If FSRMOVEND is “1” in step S129, which indicates that the SOxremoval process has been completed, the SOx removal end flag FSRMOVENDis returned to “0” (step S130), and it is determined whether or not thesecond prejudgment NG flag FNG105S is “1” (step S131). If FNG105S is“1,” it is determined that the NOx removing device 15 is deteriorated,and the program proceeds to step S134. If FNG105S is “0,” it isdetermined whether or not the second prejudgment OK flag FOK105S is “1”(step S132). If the answer to step S132 is negative (NO), this processends. If the answer to step S132 is affirmative (YES), it is determinedthat the NOx removing device 15 is normal, and the program proceeds tostep S133.

[0138] The process of FIGS. 8 and 9 is summarized as follows:

[0139] 1) If it is not determined that the SOx concentration is high bythe process of FIG. 6 (FSLF is “0”), the second prejudgment is notexecuted, but the result of the first prejudgment becomes the result ofthe deterioration determination. That is, if the downstream O2 sensoroutput LVO2 is less than or equal to the second downstream referencevalue LVO2LNCH, at the time (t14 shown in FIG. 13) the third exhaustamount parameter GALNCS (corresponding to an accumulated exhaust amountmeasured from the time the upstream O2 sensor output SVO2 has exceededthe fourth upstream reference value SVO2LNCH) has reached the thirddetermination threshold GALNCHOK, it is determined that the NOx removingdevice 15 is normal. If the downstream O2 sensor output LVO2 is greaterthan the second downstream reference value LVO2LNCH at the determinationtime t14, it is determined that the NOx removing device 15 isdeteriorated.

[0140] 2) If it is determined that the SOx concentration is high in theprocess of FIG. 6 (FSLF is “1”), the execution condition flag FMCNDF105is returned to “0” (steps S129 and S135), unless the SOx removal processhas been completed, thereby suspending the deterioration determinationprocess. That is, if the deterioration determination based on the O2sensor output is executed in the condition where the SOx concentrationis high and the high Sox concentration has a great effect on the O2sensor output, the possibility of improper determination becomes high.Accordingly, by suspending the deterioration determination in this case,the improper determination can be prevented.

[0141] If the execution condition of the deterioration determination issatisfied after completing the SOx removal process, the secondprejudgment (steps S110 to S117) is executed, and the result of thesecond prejudgment becomes the result of the deteriorationdetermination. That is, if the downstream O2 sensor output LVO2 isgreater than or equal to the fourth downstream reference value LVO2SLF,at the time (determination time tDET2 (not shown)) the fourth exhaustamount parameter GAIRLVO2 (corresponding to an accumulated exhaustamount measured from the time the downstream O2 sensor output LVO2 hasexceeded the third downstream reference value LVO2LNC) has reached thefourth determination threshold GALVO2, it is determined that the NOxremoving device 15 is normal. If the downstream O2 sensor output LVO2 isless than the fourth downstream reference value LVO2SLF at thedetermination time tDET2, it is determined that the NOx removing device15 is deteriorated.

[0142] When the SOx concentration is high and the NOx removing device 15is deteriorated, it has been experimentally confirmed that thedownstream O2 sensor output LVO2 does not reach a value indicative of anexhaust rich condition by the effect of SOx contained in the exhaustgases, even after execution of the SOx removal process for the NOxremoving device 15. Accordingly, when the downstream O2 sensor outputLVO2 is less than the fourth downstream reference value LVO2SLF at thedetermination time tDET2, it is determined that the NOx removing device15 is deteriorated.

[0143] 3) If it is determined that the downstream O2 sensor 19 hasfailed (FFSDF103 is “1”), the deterioration determination is suspended,since proper determination of deterioration cannot be expected when thedownstream O2 sensor 19 has failed. Further, if the OK determination ofthe downstream O2 sensor 19 is not made at the time t14 (or tDET2), theenrichment continuation flag FRSPEXT is set to “1” (steps S122 and S123)to extend the air-fuel ratio enrichment for the failure determinationfor the downstream O2 sensor 19.

[0144]FIGS. 10A to 10K are time charts for illustrating the operation inthe case that the SOx concentration is not determined to be high in theprocess of FIG. 6. In these time charts, the solid lines correspond tothe case where the downstream O2 sensor 19 is normal, and the brokenlines correspond to the case where the downstream O2 sensor 19 hasfailed due to short circuit (a failure such that the sensor output LVO2remains “0”).

[0145] At time t11, the execution condition of the deteriorationdetermination is satisfied and the execution condition flag FMCNDF105 isset to “1.” At time t12, the upstream O2 sensor output SVO2 exceeds thefourth upstream reference value SVO2LNCH and the measurement of thethird exhaust amount parameter GALNCS is started. At time t13, the SOxconcentration determination process (FIG. 6) is completed and theconcentration determination end flag FSLFEND is set to “1.”

[0146] At time the third exhaust amount parameter GALNCS reaches thethird determination threshold GALNCHOK (t14), the downstream O2 sensoroutput LVO2 is at the low level, so that the first prejudgment OK flagFOK105P is set to “1” (although not shown, the first prejudgment flagFPREJUD1 is also set to “1” at the same time). At this time, thedownstream O2 sensor 19 is not determined to be normal (FOKF103 is “0”),so that the enrichment continuation flag FRSPEXT is set to “1”.

[0147] At time t15, the downstream O2 sensor output LVO2 exceeds anormality determination reference value LVO2LNVH (e.g., 0.7 V), so thatthe downstream O2 sensor 19 is determined to be normal. Accordingly, theO2 sensor OK flag FOKF103 is set to “1,” and the normality flag FOKF105is set to “1”. At the same time, the enrichment continuation flagFRSPEXT is returned to “0” in the O2 sensor failure diagnosis process(not shown).

[0148] In the case where the downstream O2 sensor 19 has failed, theair-fuel ratio enrichment is further continued as shown by the brokenline. At time t16 after a predetermined time period TMRSPEXT elapsedfrom time t14, it is determined that the downstream O2 sensor 19 hasfailed (the O2 sensor failure flag FFSDF103 is set to “1”). In thiscase, the normality flag FOKF105 is maintained at “0,” that is, thenormality determination for the NOx removing device 15 is not made.

[0149]FIG. 14 is a flowchart of the SOx removal process. This process isexecuted by the CPU 5 b at predetermined time intervals (e.g., 100msec). During execution of the SOx removal process (when FSLF is “1” andFSRMOVEND is “0”), the lean operation is inhibited (see FIG. 15).

[0150] In step S151, it is determined whether or not the highconcentration flag FSLF is “1.” If FSLF is “0,” a first downcounterCSRMOV is set to a first predetermined value CTSRMOVS (e.g., 6000) (stepS153), and a second downcounter CSADINT is set to a second predeterminedvalue CTSADDS (e.g., 48) (step S154). Thereafter, this process ends. Thefirst predetermined value CTSRMOVS is set to a value corresponding tothe time during which the absorbed SOx can be completely removed, evenwhen the amount of SOx absorbed by the NOx removing device 15 is maximum(in the saturated condition).

[0151] If FSLF is “1” in step S151, which indicates that the SOxconcentration is determined to be high, it is determined whether or notthe SOx removal end flag FSRMOVEND is “1” (step S152). If the SOxremoval process has already been completed, the answer to step S152 isaffirmative (YES), and the program proceeds to step S153. If FSRMOVENDis “0,” it is determined whether or not an estimated temperature TCT ofthe NOx removing device 15 is higher than a predetermined temperatureTCTSF (e.g., 600°) (step S155). The estimated temperature TCT iscalculated in another process (not shown), e.g., by retrieving atemperature map set according to engine operating conditions,specifically, engine speed NE and engine load (absolute intake pressurePBA). Alternatively, a temperature sensor for detecting the temperatureof the NOx removing device 15 may be provided to use a detectedtemperature instead of the estimated temperature TCT.

[0152] If TCT is less than or equal to TCTSF in step S155, it isdetermined whether or not the value of the second downcounter CSADINT isless than or equal to “0” (step S158). Initially, CSADINT is greaterthan “0,” so that the second downcounter CSADINT is decremented by “1”(step S159), and the program proceeds to step S164. When the value ofthe second downcounter CSADINT becomes “0,” the program proceeds fromstep S158 to step S160, in which the first downcounter CSRMOV isincremented by “1.” Thereafter, the second downcounter CSADINT is set tothe second predetermined value CTSADDS (step S161), and the programproceeds to step S164.

[0153] If TCT is greater than TCTSF in step S155, the SOx removalenrichment flag FSRR is set to “1” to set the air-fuel ratio to a valuein the rich region with respect to the stoichiometric ratio (step S156)(see steps S10 and S29 shown in FIG. 2). Thereafter, it is determinedwhether or not the detected equivalent ratio KACT is greater than orequal to a predetermined equivalent ratio KACTSRM (e.g., 1.03) (stepS157). If KACT is less than KACTSRM, the program proceeds to step S158.If KACT is greater than or equal to KACTSRM, the first downcounterCSRMOV is decremented by “1” (step S162), and the second downcounterCSADINT is set to the second predetermined value CTSADDS (step S163).Thereafter, the program proceeds to step S164.

[0154] In step S164, it is determined whether or not the value of thefirst downcounter CSRMOV is less than or equal to the firstpredetermined value CTSRMOVS. If CSRMOV is less than or equal toCTSRMOVS, the program proceeds directly to step S166. If CSRMOV isgreater than CTSRMOVS, the first downcounter CSRMOV is set to the firstpredetermined value CTSRMOVS (step S165), and the program proceeds tostep S166.

[0155] In step S166, it is determined whether or not the value of thefirst downcounter CSRMOV is less than or equal to “0”. If CSRMOV isgreater than 0, the program ends. When the value of the firstdowncounter CSRMOV becomes “0,” it is determined that the SOx removalprocess has been completed, and steps S167 and S168 are then executed.That is, the first downcounter CSRMOV is set to “0” (step S167), and theSOx removal end flag FSRMOVEND is set to “1.” Further, the highconcentration flag FSLF is returned to “0,” and the SOx removalenrichment flag FSRR is returned to “0” (step S168). Thereafter, thisprocess ends.

[0156] According to the process of FIG. 14, the amount of SOxaccumulated in the NOx removing device 15 is estimated by the firstdowncounter CSRMOV. When the value of the first downcounter CSRMOVbecomes “0,” it is determined that the accumulated SOx has been removed,and the SOx removal end flag FSRMOVEND is set to “1.” However, when theestimated temperature TCT is less than or equal to the predeterminedtemperature TCTSF or when the detected equivalent ratio KACT is lessthan the predetermined equivalent ratio KACTSRM, SOx is not removed, butconversely accumulated into the NOx removing device 15. Accordingly,every time the value of the second downcounter CSADINT becomes “0,” thefirst downcounter CSRMOV is incremented. Since the rate of accumulationof SOx is lower than the rate of removal of SOx, the increment of thefirst downcounter CSRMOV is executed at a frequency which is lower thanthe execution frequency of the decrement.

[0157] When it is determined that the SOx concentration is high, the SOxremoval process is executed to thereby prevent improper determinationsuch that a reduction in performance of the NOx removing device 15 dueto the accumulation of SOx is improperly determined as an ageddeterioration of the NOx removing device 15.

[0158]FIG. 15 is a flowchart showing a program for inhibiting the leanoperation in which the air-fuel ratio is set in a lean region withrespect to the stoichiometric ratio, during execution of the SOx removalprocess. This program is executed by the CPU 5 b in synchronism with thegeneration of a TDC signal pulse. This process is executed immediatelyafter the lean operation permission determination process (not shown),in which the lean operation flag FLB is set according to engineoperating conditions. That is, even when the lean operation flag FLB isset to “1” in the lean operation permission determination process, thisflag FLB is returned to “0” in this process during execution of the SOxremoval process. By inhibiting the lean operation, the SOx removal canbe performed.

[0159] In step S171, it is determined whether or not the highconcentration flag FSLF is “1.” If FSLF is “1,” which indicates that theSOx concentration is determined to be high, it is determined whether ornot the SOx removal end flag FSRMOVEND is “1” (step S172). If FSLF is“1” and FSRMOVEND is “0.” which indicates that the SOx removal processis being executed, the lean operation flag FLB is set to “0” (stepS173). If FSLF is “0” or FSRMOVEND is “1,” this process ends.

[0160] In this preferred embodiment, the ECU 5 constitutes the air-fuelratio changing means, the sulfur oxide determining means, thedeterioration determining means, the inhibiting means, and the sulfuroxide removing means. More specifically, Steps S20 and S26 in FIG. 2correspond to the air-fuel ratio changing means. The process of FIG. 6corresponds to the sulfur oxide determining means. The steps S102 toS118 in FIG. 8 and the steps S127, S128, and S131 to S134 in FIG. 9correspond to the deterioration determining means. Steps S125, S129, andS135 in FIG. 9 correspond to the inhibiting means. The process of FIG.14 corresponds to the sulfur oxide removing means.

SECOND PREFERRED EMBODIMENT

[0161]FIG. 16 is a schematic diagram showing the configuration of aninternal combustion engine and a control system therefor, including anexhaust emission control system according to a second preferredembodiment of the present invention. The configuration shown in FIG. 16is different from the configuration of the first preferred embodimentshown in FIG. 1 in the point that the three-way catalyst 14 and theupstream O2 sensor 18 are not provided in the exhaust pipe 13. Thesecond preferred embodiment is similar to the first preferred embodimentexcept the following aspects.

[0162] In this preferred embodiment, the SOx concentration determinationis performed according to the output LVO2 from the downstream O2 sensor19. That is, the process of FIG. 6 is executed by using the downstreamO2 sensor output LVO2 instead of the upstream O2 sensor output SVO2 toperform the SOx concentration determination.

[0163] Further, the deterioration determination for the NOx removingdevice 15 is executed according to the output VLAF from the LAF sensor17 and the output LVO2 from the downstream O2 sensor 19. That is, thecalculation of the third exhaust amount parameter GALNCS (step S103) inthe deterioration determination subroutine (FIGS. 8 and 9) is executedby the process shown in FIG. 17.

[0164] In step S141 a in FIG. 17, it is determined whether or not theLAF sensor output VLAF is greater than or equal to a reference valueVLAFLNCH. If VLAF is greater than or equal to VLAFLNCH, the programproceeds to step S142, in which the third exhaust amount parameterGALNCS is set to “0.” If the LAF sensor output VLAF becomes lower thanthe reference value VLAFLNCH, steps S144 and S145 are executed toaccumulate the third exhaust amount parameter GALNCS.

[0165] The reference value VLAFLNCH is set to a value corresponding toan air-fuel ratio (e.g., 14.4) slightly leaner than the air-fuel ratio(about 14.3) corresponding to the deterioration determination enrichmentpredetermined value KCMDRM of the target air-fuel ratio coefficientKCMD. In this preferred embodiment, the LAF sensor output VLAF has acharacteristic such that it increases with an increase in the oxygenconcentration (an increase in the air-fuel ratio). Accordingly, by theprocess of FIG. 17, the accumulation of the third exhaust amountparameter GALNCS is started at the time the LAF sensor output VLAF haschanged from a value indicative of a lean air-fuel ratio to a valueindicative of the rich air-fuel ratio corresponding to the deteriorationdetermination enrichment predetermined value KCMDRM.

[0166] Thus, the third exhaust amount parameter GALNCS is calculatedaccording to the LAF sensor output VLAF, and the deteriorationdetermination for the NOx removing device 15 is executed similarly tothe first preferred embodiment.

[0167] In the execution condition determination process (FIGS. 4 and 5),the upstream O2 sensor output SVO2 is replaced by the LAF sensor outputVLAF, and the reference values in steps S55 and S64 are changed intovalues which is suitable for the LAF sensor output VLAF. Further, theinequality signs in steps S35 and S64 are inverted. Thus, the executioncondition for the deterioration determination can be determined by usingthe LAF sensor output VLAF.

[0168] As described above, the SOx concentration determination isexecuted according to the output from the downstream O2 sensor 19provided downstream of the NOx removing device 15 in this preferredembodiment. Further, the deterioration determination of the NOx removingdevice 15 is executed according to the LAF sensor output VLAF and thedownstream O2 sensor output LVO2.

OTHER PREFERRED EMBODIMENTS

[0169] The present invention is not limited to the above preferredembodiments, but various modifications may be made. In the firstpreferred embodiment described above, the SOx concentration isdetermined to be high if the first exhaust amount parameter GSLFFIN hasreached the first determination threshold GSLFFINR before the upstreamO2 sensor output SVO2 exceeds the second upstream reference valueSVO2SLF. A timer TSLFFIN for measuring an elapsed time period may beused in place of the first exhaust amount parameter GSLFFIN. Morespecifically, the measurement of the elapsed time period by the timerTSLFFIN may be started at the time the upstream O2 sensor output SVO2has reached the first upstream reference value SVO2LNCS, and the SOxconcentration may be determined to be high if the value of the timerTSLFFIN has reached a predetermined time period TSLFFINR correspondingto the first determination threshold GSLFFINR before the O2 sensoroutput SVO2 reaches the second upstream reference value SVO2SLF.

[0170] Further, in the first preferred embodiment described above, theaccumulation of the second exhaust amount parameter GSLFJUD is startedat the time the upstream O2 sensor output SVO2 has exceeded the secondupstream reference value SVO2SLF, and the SOx concentration isdetermined to be high if the O2 sensor output SVO2 becomes lower thanthe second upstream reference value SVO2SLF before the second exhaustamount parameter GSLFJUD reaches the second determination thresholdGASLF. A timer TSLFJUD for measuring an elapsed time period may be usedin place of the second exhaust amount parameter GSLFJUD. Morespecifically, the measurement of the elapsed time period by the timerTSLFJUD may be started at the time the upstream O2 sensor output SVO2has exceeded the second upstream reference value SVO2SLF, and the SOxconcentration may be determined to be high if the O2 sensor output SVO2becomes lower than the second upstream reference value SVO2SLF beforethe value of the timer TSLFJUD reaches a predetermined time period TASLFcorresponding to the second determination threshold GASLF.

[0171] The present invention may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.The presently disclosed embodiments are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are, therefore, to be embracedtherein.

What is claimed is:
 1. An exhaust emission control system for aninternal combustion engine having an exhaust system, comprising: NOxremoving means provided in said exhaust system of said engine forremoving NOx contained in exhaust gases when the air-fuel ratio of anair-fuel mixture to be supplied to said engine is set to a value whichis leaner than the stoichiometric ratio; an oxygen concentration sensorprovided in said exhaust system, for detecting an oxygen concentrationin the exhaust gases; air-fuel ratio changing means for changing theair-fuel ratio of the air-fuel mixture to be supplied to said enginefrom a value which is leaner than the stoichiometric ratio to a valuewhich is richer than the stoichiometric ratio; and sulfur oxidedetermining means for determining whether or not a sulfur oxideconcentration in the exhaust gases is high according to a transientcharacteristic of the oxygen concentration detected by said oxygenconcentration sensor after the air-fuel ratio is changed by saidair-fuel ratio changing means.
 2. An exhaust emission control systemaccording to claim 1, wherein said air-fuel ratio changing means changesthe air-fuel ratio from a value which is leaner than the stoichiometricratio to a value which is slightly richer than the stoichiometric ratio,and said sulfur oxide determining means determines that the sulfur oxideconcentration is high when a transient time period from the time theoxygen concentration detected by said oxygen concentration sensor hasbecome lower than a first reference value, to the time the oxygenconcentration detected by said oxygen concentration sensor reaches asecond reference value, which is less than the first reference value, islonger than a predetermined transient time period.
 3. An exhaustemission control system according to claim 1, wherein said sulfur oxidedetermining means determines that the sulfur oxide concentration ishigh, when the oxygen concentration detected by said oxygenconcentration sensor becomes lower than a concentration determinationreference value, and thereafter exceeds the concentration determinationreference value within a predetermined time period from the time theoxygen concentration becomes lower than the concentration determinationreference value.
 4. An exhaust emission control system according toclaim 1, wherein said air-fuel ratio changing means changes the air-fuelratio from a value which is leaner than the stoichiometric ratio to avalue which is slightly richer than the stoichiometric ratio.
 5. Anexhaust emission control system according to claim 1, furthercomprising: deterioration determining means for determining thedeterioration of said NOx removing means according to the output fromsaid oxygen concentration sensor; and inhibiting means for inhibitingthe deterioration determination by said deterioration determining meanswhen said sulfur oxide determining means determines that the sulfuroxide concentration is high.
 6. An exhaust emission control systemaccording to claim 1, further comprising sulfur oxide removing means forexecuting a process for removing sulfur oxide accumulated in said NOxremoving means when said sulfur oxide determining means determines thatthe sulfur oxide concentration is high.
 7. An exhaust emission controlsystem for an internal combustion engine having an exhaust system,comprising: a three-way catalyst provided in said exhaust system of saidengine for purifying exhaust gases; NOx removing means provideddownstream of said three-way catalyst for removing NOx contained in theexhaust gases when the air-fuel ratio of an air-fuel mixture to besupplied to said engine is set to a value which is leaner than astoichiometric ratio; an oxygen concentration sensor provided betweensaid three-way catalyst and said NOx removing means, for detecting anoxygen concentration in the exhaust gases; air-fuel ratio changing meansfor changing the air-fuel ratio of the air-fuel mixture from a valuewhich is leaner than the stoichiometric ratio to a value which is richerthan the stoichiometric ratio; and sulfur oxide determining means fordetermining whether or not a sulfur oxide concentration in the exhaustgases is high according to a transient characteristic of the oxygenconcentration detected by said oxygen concentration sensor after theair-fuel ratio is changed by said air-fuel ratio changing means.
 8. Anexhaust emission control system according to claim 7, wherein saidsulfur oxide determining means determines that the sulfur oxideconcentration is high, when the oxygen concentration detected by saidoxygen concentration sensor becomes lower than a first reference valueand fails to reach a second reference value which is less than the firstreference value within a predetermined time period elapsed from the timethe oxygen concentration becomes lower than the first reference value.9. An exhaust emission control system according to claim 7, wherein saidsulfur oxide determining means determines that the sulfur oxideconcentration is high, when the oxygen concentration detected by saidoxygen concentration sensor becomes lower than a concentrationdetermination reference value, and thereafter exceeds the concentrationdetermination reference value within a predetermined time period fromthe time the oxygen concentration becomes lower than the concentrationdetermination reference value.
 10. An exhaust emission control systemaccording to claim 7, wherein said air-fuel ratio changing means changesthe air-fuel ratio from a value which is leaner than the stoichiometricratio to a value which is slightly richer than the stoichiometric ratio.11. A computer program for causing a computer to carry out a method fordetermining a sulfur oxide concentration in exhaust gases from aninternal combustion engine that is provided with a NOx removing devicein an exhaust system of said engine for removing NOx contained in theexhaust gases when the air-fuel ratio of an air-fuel mixture to besupplied to said engine is set to a value which is leaner than thestoichiometric ratio, said method comprising the steps of: a) changingthe air-fuel ratio of the air-fuel mixture to be supplied to said enginefrom a value which is leaner than the stoichiometric ratio to a valuewhich is richer than the stoichiometric ratio; b) detecting an oxygenconcentration in the exhaust gases by an oxygen concentration sensorprovided in said exhaust system; and c) determining whether or not thesulfur oxide concentration in the exhaust gases is high according to atransient characteristic of the oxygen concentration detected by saidoxygen concentration sensor after changing the air-fuel ratio.
 12. Acomputer program according to claim 11, wherein the air-fuel ratio ischanged from a value which is leaner than the stoichiometric ratio to avalue which is slightly richer than the stoichiometric ratio, and thesulfur oxide concentration is determined to be high, when a transienttime period from the time the oxygen concentration detected by saidoxygen concentration sensor has become lower than a first referencevalue to the time the oxygen concentration detected by said oxygenconcentration sensor reaches a second reference value, which is lessthan the first reference value, is longer than a predetermined transienttime period.
 13. A computer program according to claim 11, wherein thesulfur oxide concentration is determined to be high, when the oxygenconcentration detected by said oxygen concentration sensor becomes lowerthan a concentration determination reference value, and thereafterexceeds the concentration determination reference value within apredetermined time period from the time the oxygen concentrationdetected by said oxygen concentration sensor becomes lower than theconcentration determination reference value.
 14. A computer programaccording to claim 11, wherein the air-fuel ratio is changed from avalue which is leaner than the stoichiometric ratio to a value which isslightly richer than the stoichiometric ratio.
 15. A computer programaccording to claim 11, wherein said method further includes the stepsof: e) determining the deterioration of said NOx removing deviceaccording to the oxygen concentration detected by said oxygenconcentration sensor; and f) inhibiting the deterioration determinationwhen the sulfur oxide concentration is determined to be high.
 16. Acomputer program according to claim 11, wherein the method furtherincludes the step of executing a process for removing sulfur oxideaccumulated in said NOx removing device when the sulfur oxideconcentration is determined to be high.
 17. A computer program forcausing a computer to carry out a method for determining a sulfur oxideconcentration in exhaust gases from an internal combustion engine havinga three-way catalyst provided in an exhaust system of said engine, forpurifying exhaust gases, and a NOx removing device provided downstreamof said three-way catalyst, for removing NOx contained in the exhaustgases when the air-fuel ratio of an air-fuel mixture to be supplied tosaid engine is set to a value which is leaner than the stoichiometricratio, said method comprising the steps of: a) changing the air-fuelratio of the air-fuel mixture to be supplied to said engine from a valuewhich is leaner than the stoichiometric ratio to a value which is richerthan the stoichiometric ratio; b) detecting an oxygen concentration inthe exhaust gases by an oxygen concentration sensor provided betweensaid three-way catalyst and said NOx removing device; and c) determiningwhether or not a sulfur oxide concentration in the exhaust gases is highaccording to a transient characteristic of the oxygen concentrationdetected by said oxygen concentration sensor after changing the air-fuelratio.
 18. A computer program according to claim 17, wherein the sulfuroxide concentration is determined to be high, when the oxygenconcentration detected by said oxygen concentration sensor becomes lowerthan a first reference value and fails to reach a second reference valuewhich is less than the first reference value within a predetermined timeperiod elapsed from the time the oxygen concentration detected by saidoxygen concentration sensor becomes lower than the first referencevalue.
 19. A computer program according to claim 17, wherein the sulfuroxide concentration is determined to be high, when the oxygenconcentration detected by said oxygen concentration sensor becomes lowerthan a concentration determination reference value, and thereafterexceeds the concentration determination reference value within apredetermined time period from the time the oxygen concentrationdetected by said oxygen concentration sensor becomes lower than theconcentration determination reference value.
 20. A computer programaccording to claim 17, wherein the air-fuel ratio is changed from avalue which is leaner than the stoichiometric ratio to a value which isslightly richer than the stoichiometric ratio.
 21. An exhaust emissioncontrol system for an internal combustion engine having an exhaustsystem, comprising: a NOx removing device provided in said exhaustsystem of said engine for removing NOx contained in exhaust gases whenthe air-fuel ratio of an air-fuel mixture to be supplied to said engineis set to a value which is leaner than the stoichiometric ratio; anoxygen concentration sensor provided in said exhaust system, fordetecting an oxygen concentration in the exhaust gases; an air-fuelratio changing module for changing the air-fuel ratio of the air-fuelmixture to be supplied to said engine from a value which is leaner thanthe stoichiometric ratio to a value which is richer than thestoichiometric ratio; and a sulfur oxide determining module fordetermining whether or not a sulfur oxide concentration in the exhaustgases is high according to a transient characteristic of the oxygenconcentration detected by said oxygen concentration sensor after theair-fuel ratio is changed by said air-fuel ratio changing module.
 22. Anexhaust emission control system according to claim 21, wherein saidair-fuel ratio changing module changes the air-fuel ratio from a valuewhich is leaner than the stoichiometric ratio to a value which isslightly richer than the stoichiometric ratio, and said sulfur oxidedetermining module determines that the sulfur oxide concentration ishigh, when a transient time period from the time the oxygenconcentration detected by said oxygen concentration sensor has becomelower than a first reference value to the time the oxygen concentrationdetected by said oxygen concentration sensor reaches a second referencevalue which is less than the first reference value, is longer than apredetermined transient time period.
 23. An exhaust emission controlsystem according to claim 21, wherein said sulfur oxide determiningmodule determines that the sulfur oxide concentration is high, when theoxygen concentration detected by said oxygen concentration sensorbecomes lower than a concentration determination reference value, andthereafter exceeds the concentration determination reference valuewithin a predetermined time period from the time the oxygenconcentration becomes lower than the concentration determinationreference value.
 24. An exhaust emission control system according toclaim 21, wherein said air-fuel ratio changing module changes theair-fuel ratio from a value which is leaner than the stoichiometricratio to a value which is slightly richer than the stoichiometric ratio.25. An exhaust emission control system according to claim 21, furthercomprising: a deterioration determining module for determining thedeterioration of said NOx removing device according to the output fromsaid oxygen concentration sensor; and an inhibiting module forinhibiting the deterioration determination by said deteriorationdetermining module when said sulfur oxide determining module determinesthat the sulfur oxide concentration is high.
 26. An exhaust emissioncontrol system according to claim 21, further comprising a sulfur oxideremoving module for executing a process for removing sulfur oxideaccumulated in said NOx removing device when said sulfur oxidedetermining module determines that the sulfur oxide concentration ishigh.
 27. An exhaust emission control system for an internal combustionengine having an exhaust system, comprising: a three-way catalystprovided in said exhaust system of said engine for purifying exhaustgases; a NOx removing device provided downstream of said three-waycatalyst for removing NOx contained in the exhaust gases when theair-fuel ratio of an air-fuel mixture to be supplied to said engine isset to a value which is leaner than a stoichiometric ratio; an oxygenconcentration sensor provided between said three-way catalyst and saidNOx removing device, for detecting an oxygen concentration in theexhaust gases; an air-fuel ratio changing module for changing theair-fuel ratio of the air-fuel mixture from a value which is leaner thanthe stoichiometric ratio to a value which is richer than thestoichiometric ratio; and a sulfur oxide determining module fordetermining whether or not a sulfur oxide concentration in the exhaustgases is high according to a transient characteristic of the oxygenconcentration detected by said oxygen concentration sensor after theair-fuel ratio is changed by said air-fuel ratio changing module.
 28. Anexhaust emission control system according to claim 27, wherein saidsulfur oxide determining module determines that the sulfur oxideconcentration is high, when the oxygen concentration detected by saidoxygen concentration sensor becomes lower than a first reference valueand fails to reach a second reference value, which is less than thefirst reference value within a predetermined time period elapsed fromthe time the oxygen concentration becomes lower than the first referencevalue.
 29. An exhaust emission control system according to claim 27,wherein said sulfur oxide determining module determines that the sulfuroxide concentration is high, when the oxygen concentration detected bysaid oxygen concentration sensor becomes lower than a concentrationdetermination reference value, and thereafter exceeds the concentrationdetermination reference value within a predetermined time period fromthe time the oxygen concentration becomes lower than the concentrationdetermination reference value.
 30. An exhaust emission control systemaccording to claim 27, wherein said air-fuel ratio changing modulechanges the air-fuel ratio from a value which is leaner than thestoichiometric ratio to a value which is slightly richer than thestoichiometric ratio.